Detection of targets by radars, especially by airborne radars, is very difficult because the vegetative "clutter" which surrounds the targets can mask targets or cause the radar to misidentify targets. "Clutter" is generally described as radar reflections from natural objects, like vegetation. Clutter is different from noise, although both interfere with target detection. Noise generally refers to a random distribution of unwanted thermal energy.
Target detection is particularly difficult for radars operating at millimeter wavelengths. Such radars, which are often used in tactical missiles, attempt to locate targets which are hidden by heavy vegetative clutter. To improve target detection, many millimeter radars contain moving target indicators. If the targets are not moving, however, they may easily avoid detection.
Conventional radar signal processors cannot selectively reduce noise and clutter signals to enhance a radar's ability to detect both fixed and moving targets. The most common radar system for target detection employs a coherent radar which integrates several radar returns from a single antenna beam position. The integration results in the coherent addition of returns from targets and the reduction of noise energy. Much of the radar energy reflected from vegetative clutter, however, is not canceled by the integration process. Thus, while this method may improve the signal to noise ratio, it does not improve the signal-to-clutter ratio significantly. Furthermore, since the process of coherent integration on a moving platform requires motion compensation, systems which use this method are difficult to implement.
Another approach to the problem of target detection employs Doppler Beam Sharpening (DBS). DBS systems take advantage of the fact that slightly different doppler frequencies occur across the antenna beamwidth of a moving radar. Beam sharpening occurs by selecting only a portion of these doppler frequencies for analysis, since the signals used come from only a narrow portion of the antenna azimuth extent. When the azimuth extent of the beam is reduced, the amount of energy due to clutter is also reduced. The practical realization of a DBS system is very complex, since it depends on missile velocity and requires a sideways look angle. Thus, DBS systems can be adversely affected by changes in missile velocity, and the use of DBS systems may limit missile maneuverability.
Other techniques used for radar signal processing include two- and three-pulse clutter cancelers and moving target indicators. These techniques are undesirable since they require a coherent radar, rely on target motion, and are sensitive to changes in target velocity. Furthermore, such systems are not sensitive to very low target velocities of only a fraction of a meter per second and cannot detect fixed targets surrounded by vegetation.
An object of the present invention therefore is a radar signal processor with very fast and accurate target detection and classification.
Another object of this invention is a fast and accurate radar signal processor which can be used in a missile without being significantly affected by missile velocity.
A further object of the present invention is a radar signal processor which can detect both stationary and moving targets.
Additional objects and advantages of the invetion will be set forth in the description which follows or may be learned by practice of the invention.